The present invention relates to the art of oxygen concentration measurement and control and more particularly to method and apparatus for measuring and controlling the concentration of oxygen in combustion exhaust gases.
Industrial furnaces often include oxygen sensors located within the flue of the furnace for sensing the concentration of oxygen in the flue. These measurements are used to control the rate at which fuel and/or oxidizer is supplied to the furnace. Commonly, these oxygen sensors are comprised of zirconium dioxide formed into a generally test tube like shape. The closed end of the sensor is inserted within the flue of the furnace, whereas the open end is exposed to a gas having a known oxygen concentration (usually air). Electrodes were plated onto the inner and outer surfaces of the tube, and a voltage measuring device connected across these electrodes. If the oxygen concentrations on the two sides of the sensor are different, oxygen ions will diffuse through the sensor wall from the lower concentration area to the higher concentration area. The diffusion of oxygen through the sensor wall gives rise to an electrical potential across the electrodes, where the magnitude of the potential is related to the rate of oxygen flow and hence to the concentration of oxygen within the flue. This electrical potential is thus useful as an indication of the concentration of oxygen within the flue.
Unfortunately, these zirconium dioxide oxygen sensors are also sensitive to temperature changes. In other words, the magnitude of the electrical potential at the sensor output is dependent not only on the concentration of oxygen within the flue, but also upon the temperature of the sensor. Consequently, the sensor output cannot be used as a direct measure of oxygen concentration unless either the temperature of the sensor is stabilized at a known temperature or else the temperature of the sensor is taken into consideration in converting the electrical potential into an oxygen measurement. In either event it is necessary to determine the temperature of the sensor.
One prior art method of determining the temperature of the sensor is to fabricate the zirconium dioxide oxygen sensor with an integral thermocouple, whereby the thermocouple can be used to provide a direct measurement of the temperature of the sensor. Unfortunately, in such sensors it is necessary to replace the entire sensor whenever the thermocouple becomes faulted. Since ziconium dioxide oxygen sensors tend to be rather expensive (the electrodes which are plated onto the inner and outer surface of the sensor are generally platinum) this is not very desireable. Other systems have placed the thermocouple adjacent to but not integral with the sensor body itself. Patents describing sensors of this nature include the patents to Sayles, U.S. Pat. No. 3,546,086, Wilson, U.S. Pat. No. 3,720,594 and McIntyre et al., U.S. Pat. No. 3,928,161.
It is known that the resistance of the sensor changes with temperature. This characteristic has been used in the past in the determination of the temperature of the sensor. In one system an external DC electrical potential is applied to the sensor electrodes through a known resistance, with the magnitude of the resulting DC potential appearing across the sensor being used as an indication of the resistance of the sensor and, thus, the temperature thereof. Since a DC voltage is used, it is not possible to distinguish between the potential induced by the externally applied DC potential and the actual sensor output; both are DC signals. Therefore, in this system the two functions (oxygen measurement and temperature measurement) are performed sequentially rather than simultaneously. The DC output signal produced by the sensor due to the oxygen concentration within the flue is first sampled, with the amplitude of this DC signal being stored for later use. During this measurement the external DC potential is switched off so as not to perturb the output reading. Only after this is the external DC potential applied to the sensor for measurement of the sensor resistance. The DC potential then appearing across the sensor is corrected by subtracting the DC oxygen concentration signal previously measured. In this fashion, each of the two tests could be conducted without interfering with the other.
The temperature measurement derived by determining sensor resistance is then used to control the operation of a heater which maintains the temperature of the sensor within a predetermined range. Since the temperature of the sensor is still permitted to vary within this range, however, the sensor output can still be expected to vary somewhat in accordance with the temperature of the sensor. In order to avoid this, one set-point controller, known as the "Optimizer" and manufactured and sold by the assignee of this application, utilizes a reference gas having an oxygen concentration which is identical to the desired oxygen concentration in the flue. Since the output of the sensor will always be zero (regardless of temperature) when the oxygen concentration inside and outside of the sensor are the same, this simple expedient eliminates temperature variations at the set point. When the oxygen concentration in the flue differs from the desired or set point concentration, however, temperature induced deviations can still occur. This is not important in the Optimizer controller because the only point of interest is the set-point concentration; the flue oxygen concentration is never actually measured. If the output of the sensor were used for oxygen concentration measurement rather than merely set-point control, however, these temperature deviations would become troublesome.